The expression of the cloned ovalbumin gene will be studied by transferring it to a hormonally responsive cell. The vextor for the transfer is a temperature-sensitive thymidine kinase gene which we have cloned. The ovalbumin gene will be linked to the temperature-sensitive kinase gene and transferred to a mammalian cell with a functional estrogen receptor. This will then allow the studies of steroid mediated gene activation in a defined in vitro system.